1. Field of the Invention
The present invention relates to a switching power-supply circuit that outputs a voltage from each of a plurality of output portions, and in particular, to a switching power-supply circuit including a circuit that stabilizes the output voltage of a second output portion, in addition to a feedback circuit that feedback-controls a primary side in accordance with the output voltage of a first output portion.
2. Description of the Related Art
In International Publication No. WO2006/061924, Japanese Unexamined Patent Application Publication No. 3-7062, and Japanese Unexamined Patent Application Publication No. 2000-217356, switching power-supply circuits are disclosed, each of which outputs a voltage from each of a plurality of output portions. In each of International Publication No. WO2006/061924, Japanese Unexamined Patent Application Publication No. 3-7062, and Japanese Unexamined Patent Application Publication No. 2000-217356, a configuration is used in which a rectified voltage or a rectified and smoothed voltage is input to output a second output (sub output) voltage separately from the first output (main output) of a converter and the second output voltage is adjusted by controlling a switching element on a second output side in accordance with this second output voltage.
In a switching power-supply circuit disclosed in International Publication No. WO2006/061924, a time period when electric power is supplied from the primary side of a transformer to the secondary side thereof is set based on a resonant frequency that is defined by a capacitor used for current resonance and the leakage inductance of the transformer (See, for example, Paragraph [0034] in International Publication No. WO2006/061924). In addition, in a second output line, an output control switching element and an output control circuit controlling the pulse widths of the turn-on and turn-off thereof are provided (See, for example, Paragraph [0028] in International Publication No. WO2006/061924).
FIG. 1A is the schematic view of the switching power-supply circuit illustrated in International Publication No. WO2006/061924, and FIG. 1B is the block diagram of the inside of the output control circuit. In the switching power-supply circuit, an output control MOS-FET 40 is connected between the cathode of a second output rectifier diode 15 and a second output smoothing capacitor 16 which are included in the second rectifying/smoothing circuit 17, and between second direct-current output terminals 18, 19 and the gate of the output control MOS-FET 40, an output control circuit 41 is provided to control the turn-on and turn-off of the output control MOS-FET 40 based on the voltage of the second output smoothing capacitor 16. The output control MOS-FET 40 is subjected to an on-off operation in synchronization with the on-period of a first main MOS-FET 1 and with a same switching frequency. In addition, a main control circuit 14 fixes the on-period of the first main MOS-FET 1, and controls the on-duty of the first main MOS-FET 1 by changing the on-period of a second main MOS-FET 2 based on the output voltage Vo1 of a first rectifying/smoothing circuit 9.
As illustrated in FIG. 1B, the output control circuit 41 includes a voltage fluctuation detection circuit 42 detecting a voltage Vt22 occurring in the second secondary winding 5c of a transformer 5 at the time of the turn-on of the first main MOS-FET 1, a second output voltage detection circuit 43 detecting the voltage Vo2 of the second output smoothing capacitor 16 and outputting an error signal Ve2 between the detected voltage and a reference voltage specifying a second output voltage value, a PWM control circuit 44 that is driven by the detection signal Vtd of the voltage fluctuation detection circuit 42 and outputs a pulse train signal Vpt having a duty ratio controlled based on the error signal Ve2 of the second output voltage detection circuit 43, an RS flip-flop 45 that is set by the detection signal Vtd of the voltage fluctuation detection circuit 42 and reset by the pulse train signal Vpt of the PWM control circuit 44, and a drive circuit 46 supplying an actuation signal Vs2 to the gate of the output control MOS-FET 40 in response to the output signal of the RS flip-flop 45.
With this configuration, the output control MOS-FET 40 is PWM-controlled in accordance with the second output voltage Vo2, and the second output voltage Vo2 is stabilized.
In a switching power-supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 3-7062, a configuration is provided in which a first output voltage is controlled by a frequency modulator including a secondary-side control mechanism controlling the duty cycle of a pulse train and a second output voltage is controlled by thinning the number of pulse voltage waves.
In a switching power-supply circuit disclosed in Japanese Unexamined Patent Application Publication No. 2000-217356, a switch circuit is provided in the secondary winding output of a transformer in an output system other than an output system performing main feedback. Furthermore, an output voltage is detected, the ON-width of the secondary winding output of the transformer in the output system other than the output system performing the main feedback is controlled by causing a pulse width control circuit to generate a pulse signal and causing this pulse signal to be synchronized with the pulse control signal of the output system performing the main feedback, and the output voltage is stabilized.
In each of the switching power-supply circuits disclosed in International Publication No. WO2006/061924, Japanese Unexamined Patent Application Publication No. 3-7062, and Japanese Unexamined Patent Application Publication No. 2000-217356, a PWM control circuit is required that is used to generate a pulse train signal having a duty ratio based on the error signal of a second output voltage detection circuit, and the overall control circuit is complicated.